1. Field of the Invention
The present invention relates generally to a handover apparatus and method in a wireless communication system, and in particular, to a handover apparatus and method in a heterogeneous wireless communication system.
2. Description of the Related Art
Wireless communication systems have been developed to allow users to communicate, regardless of location. Such wireless communication systems are evolving from early systems that provide only simple voice communication services into advanced systems capable of providing high-speed data services, packet-based Voice-over-Internet Protocol (VoIP) services, etc.
Wireless communication systems are increasingly being developed, from Circuit Switching (CS) systems that provide basic voice services and lower-speed data services, into advanced systems that provide high-speed packet data services. Among the wireless communication systems, CS-based systems include the 2nd Generation (2G) system and the 3rd Generation (3G) system. The 3G system can provide higher-speed data services than the 2G system. However, due to a need for a system more advanced than the 2G or 3G systems in order to meet the increasing user demand for super high-speed data services, research and development is being conducted on the Long Term Evolution/Service Architecture Evolution (LTE/SAE) system.
The wireless communication systems provide services through a plurality of base stations since they should secure mobility of users. Therefore, handover from a particular base station to another base station may frequently occur while the users do not perceive the handover. For example, if a user receiving a service from a particular base station communicates while moving, the user will continuously receive the service from another base station during and after handover. The procedure for transferring the ongoing call from a particular base station to another base station regardless of movement of the user is called ‘handover’. This handover technology is a very common technology in the wireless communication systems.
While the currently available wireless communication systems are 2G/3G-based systems, future available wireless communication systems will be LTE/SAE-based systems. Generally, all of the wireless systems cannot be replaced simultaneously, i.e., it is very difficult to replace all base stations and their upper layer nodes from the 2G/3G system to the LTE/SAE system.
Therefore, there is a possible case where the 2G/3G system and the LTE/SAE system may be used together. Even in this case, handover should be applied to all users in common, to remove inconveniences to users. However, the 2G/3G system provides the voice services using the CS scheme. The CS scheme assigns a fixed communication line between users desiring to make a voice call. That is, in the CS scheme, a voice signal is transmitted through a CS call. On the other hand, the LTE/SAE system is a network where all services use Packet Switching (PS) scheme. Therefore, the LTE/SAE system provides voice services based on VoIP using an IP Multimedia Subsystem (IMS).
In such cases where different systems coexist, the handover scheme from the LTE/SAE system to the CS system or from the CS system to the LTE/SAE system can be roughly classified into a scheme of performing handover so as to allow a user to receive signals from different networks using two radio channels, and a scheme of performing handover using only one radio channel. Both of the two schemes need a method for switching the existing call to another call in order to provide handover of a terminal in the system.
The two-radio channel based scheme should provide each terminal with its own associated radio channel elements, causing an increase in the cost of the terminal. From the standpoint of the terminal, the one-radio channel based scheme is more efficient than the two-radio channel based scheme. However, no method has been proposed yet in which a terminal using only one radio channel supports handover. Therefore, there is a demand for a method for performing handover to the CS system in a terminal that uses only one radio channel and is receiving a call from the LTE/SAE system.